[The acromio-humeral interval. A study of the factors influencing its height]

The narrowing of the Acromio Humeral Interval (AHI) under 6-7 mm, lower limit reported in normal shoulder, has been considered to be a specific indicator for full-thickness cuff tears. The purpose of this study was to analyse the factors influencing the AHI.

264 shoulders were operated on between 1984 and 1994 for full thickness tear of the supraspinatus and infraspinatus associated or not with an anterior cuff lesion. All patients had X-ray with A.P. view of the shoulder in neutral rotation. The AHI was defined as the shortest distance measured between the inferior cortex (dense line) of the acromion and the humerus. CT arthrogram was performed in 84 patients to analyse cuff muscular fatty degeneration. A full thickness tear was confirmed and measured by surgical approach in all cases.

There was a moderate significant relationship (p < 0.05) between AHI and symptoms duration. There was a significant relationship between AHI and tear size (p < 0.05). When the supraspinatus tendon was only torn, the mean AHI was 9.5 +/- 0.17 mm (4.5 per cent of narrowing AHI). When supra and infraspinatus tendon were torn, the mean AHI was 7.5 +/- 0.4 mm (28 per cent of narrow AHI) and when an anterior lesion was associated, the mean AHI decreased to 5.4 +/- 0.5 mm and the percentage of narrow AHI increased to 63 per cent. The AHI was not influenced by the biceps rupture: with a similar symptoms duration, the mean AHI was 9 +/- 0.5 mm with biceps tear versus 9.2 +/- 2 mm when the biceps was normal. However, the mean AHI decreased to 5.5 +/- 0.6 mm when the long head of the biceps was dislocated and the AHI was less than 7 mm in 61 per cent of the cases. A highly significant relationship was found between the AHI and the infraspinatus muscle degeneration. We have found 100 per cent incidence of AHI narrowing when the infraspinatus was degenerated (mean AHI 2.2 +/- 1.1 mm). A moderate similar relationship was found concerning the supraspinatus muscle (p < 0.05) and no relationship was found with the subscapularis degeneration.

The infraspinatus, external rotator of the humerus, seems to be the major active depressor of the humeral head. The biceps tendon, which is a passive depressor of the humerus, has no influence on the AHI whatever it is ruptured or not. However biceps dislocation is associated with significant humeral head superior migration. Symptoms duration and cuff tear size seem to be only secondary factors affecting the AHI.

AHI narrowing should evocate a severe cuff tear with biceps dislocation or muscular degeneration. In these cases, surgical repair might be questionable. AHI value is more prognostic than diagnostic.